South Beach, Atkins, and their ilk offer modest results in the short-term.

Action Points

Although millions of Americans attempt to lose weight each year by following popular commercial diets (Atkins, South Beach, Weight Watchers), a systematic review found little difference between the four with regard to weight loss and cardiovascular health.

Note that limited 24-month data suggested that weight lost was partially regained over time.

Atkins, South Beach, Weight Watchers, the Zone -- millions of Americans attempt to lose weight each year by following one of these commercial diets, but a systematic review found little difference between the four with regard to weight loss and cardiovascular health.

The authors analyzed the efficacy of the for weight-loss plans with a focus on sustained weight loss at ≥12 months.

They found that, at 12 months, the 10 randomized trials comparing popular diets with usual care revealed that only Weight Watchers was consistently more efficacious for reducing weight, with a range of mean changes from minus 3.5-minus 6.0 kg (about 8-13 lbs) versus minus 0.8-minus 5.4 kg (P<0.05 for three of four randomized trials), reported Renee Atallah, MSc, of McGill University in Montreal, and colleagues.

Limited 24-month data suggested that weight lost was partially regained over time, they added.

"Our results suggest that all four diets are modestly efficacious at decreasing weight in the short-term, but that these benefits are not sustained long-term," Atallah told MedPage Today.

Atallah said the review focused on the four diet plans because they are followed by so many people in North America and worldwide.

"We felt it was important to include these four diets because they form a representative sample of the commercial, popular diets that are available to people who want to lose weight," she told MedPage Today. "More precisely, the three book-based diets (Atkins, South Beach, and Zone) have all achieved bestseller status, and Weight Watchers, another leading popular diet with its millions of members worldwide, offered an interesting contrast with its meeting-based approach."

By one estimate, people in the U.S. spent more than $66 billion on weight loss in 2013, but the efficacy and health risks and benefits of the most popular commercial diets remain the subject of much speculation.

Diet Details

Three of the diets included in the analysis -- Atkins, South Beach, and Zone -- emphasize limiting carbohydrates to varying degrees.

Atkins is a four-phase diet that eliminates carbohydrates in the first phase, but with unlimited protein and fat.

South Beach is a three-phase modified low carbohydrate, high protein diet that emphasizes lean proteins, mono- or polyunsaturated fats, and low-glycemic index carbohydrates.

Zone is a low-carbohydrate diet based on low-fat proteins, low-glycemic load carbohydrates and small amounts of monounsaturated fats such as olive oil and avocado.

Weight Watchers is a meeting-based commercial diet plan that stresses behavior modification and physical activity using a personalized point system to track caloric intake.

Studies Fall Short

In an effort to compare the efficacy of the four diet strategies, Atallah and colleagues conducted a systematic search of MEDLINE, EMBASE, and the Cochrane Library from inception to May of 2014 to identify randomized controlled trials published in English that examined their effects on weight loss and cardiovascular risk.

Their search initially yielded 8,393 potentially relevant trials, and after screening titles and abstracts, 645 were reviewed. Of those trials, just 26 randomized controlled trials met inclusion criteria, and 14 of these had follow-up of less than 12 months.

Most of the 26 randomized trials had unclear risk of bias in the domains of allocation concealment, blinding, and incomplete data reporting, and several trials had incomplete outcome collection and a high rate of loss to follow up.

"Included studies had high attrition rates, and studies varied in their statistical approach to address this limitation," the researchers wrote.

One of the key messages from the analysis is that better studies are needed to understand the short- and long-term efficacy of the four diets, Atallah said.

"We need more head-to-head trials directly comparing these four diets, and involving a large number of participants and research sites, in order to get the best picture of which diet strategy is best for weight loss," she said.

She added that there is even less known about the impact of the four diets on key aspects of cardiovascular health, including lipid profile, blood pressure and glycemic control.

Diet Plans and Heart Health

Of the 14 short-term (<12 months) randomized trials, three examined the effect of South Beach, Weight Watchers, and Zone on lipid profiles. Eight provided data on Atkins and suggested a favorable impact with this diet on HDL cholesterol and triglycerides relative to usual care, Weight Watchers, and Zone. The limited data suggested that Atkins may have an adverse effect on LDL cholesterol levels, with unchanged or increased levels observed in five trials.

Long-term (≥12 months) randomized trials comparing Atkins with usual care showed significant improvements in HDL cholesterol and, to a lesser extent, triglyceride levels at 12 and 24 months, with no evidence of a LDL cholesterol increase. There were no or limited data on the effect of South Beach and Weight Watchers on lipid profiles versus usual care at ≥12 months.

Findings from the short-term trials suggested that Atkins and Weight Watchers had favorable effects on systolic blood pressure and diastolic blood pressure. No blood pressure data were available for the South Beach diet and only one trial included data on Zone.

In the long-term trials, Weight Watchers was found to have inconsistent long-term effects of blood pressure versus usual care. But a significant improvement in diastolic blood pressure at 12 months was shown with Weight Watchers in a 2-year follow up trial published in 2003. Atkins also showed a significant improvement in diastolic blood pressure (percent change minus 3.7) in another trial reported in 2003.

Findings from the two head-to-head trials suggested that Zone was less effective than Atkins and Weight Watchers and similar to usual care for improving systolic and diastolic blood pressure.

Modest Evidence of Benefit

No major differences in glycemic control were seen between the diets in the short-term randomized trials. Atkins showed a trend toward worsening fasting glucose, but improved fasting insulin and there were limited data available on Weight Watchers and Zone (one trial each). The long-term data were also limited, but findings from the head-to-head trials suggest that Atkins, Weight Watchers, Zone, and control were similar for improving fasting glucose and insulin at 12 months.

"Despite their popularity and important contributions to the multi-million dollar weight loss industry, we still do not know if these diets are effective to help people lose weight and decrease their risk factors for heart disease," noted senior author Mark J. Eisenberg, MD, MPH, of Jewish General Hospital/McGill University in a press statement.

"With such a small number of trials looking at each diet and their somewhat conflicting results, there is only modest evidence that using these diets is beneficial in the long term," he said, adding that larger clinical trials with longer follow up times are needed.